The present invention relates to an image forming device which is applied to a printing unit of digital copying machines or facsimiles, or digital printers, plotters and the like for forming images upon a recording medium by flying a developing agent.
A number of prior art image forming devices have adopted, so-called xe2x80x9celectrostatic photographic or xerographic process in which image information is converted into photo-information, which is incident upon a photosensitive material, on which electrostatic latent images are formed and then the images are developed by a developing agent. Recently, image forming device which directly flies an ink or developing agent for conducting a high definition image forming with a simpler arrangement has been proposed since rapid digitalization of the image forming device has been advanced due to wide spreading of computers and their advancement in performance. Since printing of visually excellent high definition, which is equivalent to that of electrostatic photography can be conducted by using a developing agent can be conducted in the image forming device in which a developing agent is directly flown and the necessity of any optical writing system or photosensitive material is obviated, a number of image forming systems have been proposed. A process in which images of a developing agent are formed by applying a voltage upon wires which are formed in a matrix to form electrostatic images in the vicinity of the wires and applying a developing agent upon the electrostatic images is disclosed in, for example Japanese Patent Publication No. 1(1989)-503221.
In such an image forming device, the developing agent is attracted to a developing agent carrier by various attracting forces such as electrostatic, intermolecular, liquid crosslinking forces. It is necessary to apply upon the developing agent a high electrostatic field of several MV/m or more enough to overcome these attracting forces in order to fly the developing agent. Several control electrodes and a high voltage switching device are required to control the flight of the developing agent. The breakdown voltage of the high voltage ICs is about 300 V. It is necessary to preset the distance between the developing agent carrier and the control electrodes to about 100 xcexcm in order to provide a strength of electric field of several MV/m and it is necessary to strictly keep the distance between the developing agent carrier and the control electrodes at an accuracy in order of xcexcm. If such accuracy is not kept, a local ununiformity of flight electric field strength may occur, resulting in variations in the amount of flown developing agent.
In order to solve the problems of the prior art, various concepts have been proposed for supplying a developing agent in the form of cloud to overcome the attracting force between the developing agent and the developing agent carrier.
A concept in which the developing agent is formed into cloud form by using a brush-like developing agent carrier and by tapping the brush unit with a blade is disclosed in Japanese Laid-Open Patent Publication No. 3(1991)-215874.
Concepts for forming the developing agent into a cloud form by using a belt-like developing agent carrier and in position opposite to the control electrodes, by contacting a cam-like member to the reverse surface of the belt, or by imparting ultrasonic vibration and electrical vibration thereto are disclosed in Japanese Laid-Open Patent Publication Nos. 4(1992)-168064, 4(1992)-238050 and 5(1993)-131671.
However, if the brush-like developing carrier is used, problems may occur in which control of the amount of conveyed developing agent is ununiform and in which the cloud form is changed by bending of brush fibers due to aging. If belt-like developing agent carrier is used, imparted vibration is conducted through the belt to give an adverse influence upon a developing agent layer forming unit, or to change the position of the belt per se, which faces the control electrodes, so that the flying electric field is changed to give an adverse influence upon the amount of flown developing agent. Furthermore, it is hard to keep a uniform and stable cloud condition only by imparting such mechanical vibration, so that an aggregate of the developing agent remains. This may cause a problem in that this aggregate will be deposited to the electrodes and clog openings.
In order to overcome such a problem in Japanese Laid-Open Patent Publication No. 5(1993)-330126, a process in which after cloud condition is established as is disclosed in Japanese Laid-Open Patent Publication No. 3(1991)-215874, the aggregate of the developing agent is crushed by forming an oscillating electric field between a pair of electrodes which sandwich the flight path of the developing agent therebetween to further continue the cloud condition.
However, even in such a process, a mechanism to cause the cloud condition to occur is unstable as mentioned above and it is hard to stably obtain a desired effect even if an oscillating electric field is formed. The electrostatic charges of the developing agent may be specifically distributed. It has been found that if formation of the oscillating electric field when an inappropriate condition will conversely lower the density of the cloud, resulting in partial lowering of the image density. It has been also found that if the voltage application condition is inappropriate in an arrangement to establish oscillating electric fields which sandwich the flight path of the developing agent therebetween, a velocity component which is normal to the flight direction would be imparted to the developing agent, so that spreading of the developing agent may occur in the course of the flight of the developing agent, giving an adverse influence upon the image forming process.
An object of the present invention is to provide an image forming device in which a developing agent is stably supplied in the cloud form at a high density and image forming is conducted by controlling flight/non-flight of the developing agent at a low voltage and the influence of the accuracy of the position of the control electrodes is reduced.